Implement fast multi-item append

[georgematheos]

Currently append is implemented by inserting one item at a time; I think it should be possible to do this faster by adding all elements "at once".

[schrauf]

That would be great, just should be careful if using append!(a::MutableLinkedList,b::MutableLinkedList) from DataStructures.jl as it currently can generate segfaults and infinite loops.
Issue #725